<template>
	<transition :name="transitionName">
		<div v-show="visible" :style="customStyle" class="back-to-ceiling" @click="backToTop">
			<svg width="16" height="16" viewBox="0 0 17 17" xmlns="http://www.w3.org/2000/svg" class="Icon Icon--backToTopArrow"
				aria-hidden="true" style="height: 16px; width: 16px;">
				<title>回到顶部</title>
				<g>
					<path
						d="M12.036 15.59c0 .55-.453.995-.997.995H5.032c-.55 0-.997-.445-.997-.996V8.584H1.03c-1.1 0-1.36-.633-.578-1.416L7.33.29c.39-.39 1.026-.385 1.412 0l6.878 6.88c.782.78.523 1.415-.58 1.415h-3.004v7.004z"
						fill-rule="evenodd" />
				</g>
			</svg>
		</div>
	</transition>
</template>

<script>
	export default {
		name: 'BackToTop',
		props: {
			visibilityHeight: {
				type: Number,
				default: 400
			},
			backPosition: {
				type: Number,
				default: 0
			},
			customStyle: {
				type: Object,
				default: function() {
					return {
						right: '50px',
						bottom: '50px',
						width: '40px',
						height: '40px',
						'border-radius': '4px',
						'line-height': '45px',
						background: '#e7eaf1'
					}
				}
			},
			transitionName: {
				type: String,
				default: 'fade'
			}
		},
		data() {
			return {
				visible: false,
				interval: null,
				isMoving: false
			}
		},
		mounted() {
			window.addEventListener('scroll', this.handleScroll)
		},
		beforeDestroy() {
			window.removeEventListener('scroll', this.handleScroll)
			if (this.interval) {
				clearInterval(this.interval)
			}
		},
		methods: {
			handleScroll() {
				this.visible = window.pageYOffset > this.visibilityHeight
			},
			backToTop() {
				if (this.isMoving) return
				const start = window.pageYOffset
				let i = 0
				this.isMoving = true
				this.interval = setInterval(() => {
					const next = Math.floor(this.easeInOutQuad(10 * i, start, -start, 500))
					if (next <= this.backPosition) {
						window.scrollTo(0, this.backPosition)
						clearInterval(this.interval)
						this.isMoving = false
					} else {
						window.scrollTo(0, next)
					}
					i++
				}, 16.7)
			},
			easeInOutQuad(t, b, c, d) {
				if ((t /= d / 2) < 1) return c / 2 * t * t + b
				return -c / 2 * (--t * (t - 2) - 1) + b
			}
		}
	}
</script>

<style scoped>
	.back-to-ceiling {
		position: fixed;
		display: inline-block;
		text-align: center;
		cursor: pointer;
	}

	.back-to-ceiling:hover {
		background: #d5dbe7;
	}

	.fade-enter-active,
	.fade-leave-active {
		transition: opacity .5s;
	}

	.fade-enter,
	.fade-leave-to {
		opacity: 0
	}

	.back-to-ceiling .Icon {
		fill: #9aaabf;
		background: none;
	}
</style>